Diesel aftertreatment system

ABSTRACT

A diesel exhaust gas aftertreatment system ( 10 ) is provided to treat the exhaust ( 12 ) from a diesel combustion process ( 14 ), such as a diesel compression engine ( 16 ). The system ( 10 ) includes a burner ( 18 ) that selectively supplies the exhaust ( 12 ) at an elevated temperature to the rest of the system ( 10 ), a diesel particulate filter ( 20 ) connected downstream from the burner ( 18 ) to receive the exhaust ( 12 ) therefrom, and a NO x  reducing device ( 22 ) connected downstream from the filter ( 20 ) to receive the exhaust therefrom.

CROSS-REFERENCE TO RELATED APPLICATIONS

Not Applicable.

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable.

MICROFICHE/COPYRIGHT REFERENCE

Not Applicable.

FIELD OF THE INVENTION

This invention relates to systems and methods for treating exhaust gases from a diesel combustion process, such as a diesel compression engine, and more particularly to systems for reducing oxides of nitrogen (NO_(x)) and particulate matter (PM) emissions from diesel compression engines.

BACKGROUND OF THE INVENTION

Environmental regulations have called for increasing emission limits that require reduction in the NO_(x) and PM from diesel combustion processes, and in particular from diesel compression engines. While diesel particulate filters (DPF) are capable of achieving the required reductions in PM, which is typically carbonaceous particulates in the form of soot, there is a continuing need for improved systems that can provide the required reductions in NO_(x) in connection with the particulate matter reduction provided by a DPF.

In this regard, systems have been proposed to provide a diesel oxidation catalyst (DOC) upstream from a DPF in order to provide an increased level of NO₂ in the exhaust which reacts with the soot gathered in the DPF to produce a desired regeneration of the DPF (often referred to as a passive regeneration). However, such systems become limited at temperatures below 300° C. and typically produce a pressure drop across the oxidation catalyst that must be accounted for in the design of the rest of the system. Additionally fuel, such as hydrogen or hydrocarbon fuel, can be delivered upstream of the DOC to generate temperatures greater than 600° F. in the DPF (often referred to as active regeneration).

SUMMARY OF THE INVENTION

In accordance with one feature of the invention, a diesel exhaust gas treatment system is provided to treat the exhaust from a diesel combustion process. The system includes a burner to receive the exhaust and selectively heat the exhaust with a flame to supply the exhaust at an elevated temperature to the rest of the system, a diesel particulate filter (DPF) connected downstream from the burner to receive the exhaust therefrom, and at least one of a selective catalytic reduction catalyst (SCR) and a NO_(x) trap connected downstream from the diesel particulate filter to receive the exhaust therefrom.

As one feature, the system further includes a diesel oxidation catalyst connected downstream from the burner to receive the exhaust therefrom and upstream from the DPF to deliver the exhaust thereto. In a further feature, the system further includes a fuel injector located downstream from the burner and upstream of the DOC.

In one feature, the system further includes a diesel oxidation catalyst connected downstream from the DPF to receive the exhaust therefrom and upstream from the SCR to deliver the exhaust thereto.

According to one feature, the burner includes at least one fuel injector and at least one igniter.

As one feature, the at least one of a selective catalytic reduction catalyst and a NO_(x) trap is a selective catalytic reduction catalyst and further includes a reductant injector connected upstream from the catalyst.

In accordance with one feature of the invention, a method is provided for treating a diesel exhaust from a diesel combustion process. The method includes the steps of:

(a) selectively increasing the temperature of the exhaust by burning a fuel in the exhaust flow downstream from the diesel combustion process;

(b) removing soot from a filter by oxidizing carbon into the increased temperature exhaust provided from step (a); and

(c) removing NO_(x) carried in the exhaust provided from step (b).

In one feature, the method further includes the step of producing NO₂ by passing the exhaust from step (a) through an oxidation catalyst prior to step (b). As a further feature, the method further includes the step of injecting fuel into the exhaust after step (a) and prior to step (b).

In a further feature, the method of further includes the step of producing NO₂ by passing the exhaust from step (b) through an oxidation catalyst prior to step (c).

According to one feature, step (a) includes the steps of injecting a fuel into the exhaust and igniting the fuel.

As one feature, step (c) includes converting NO_(x) to N₂ by passing the exhaust over a selective catalytic reduction catalyst.

In one feature, step (c) includes trapping NO_(x).

Other objects, features, and advantages of the invention will become apparent from a review of the entire specification, including the appended claims and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic representation of a diesel exhaust gas treatment system embodying the invention in connection with a diesel combustion engine; and

FIGS. 2-4 are a representations similar to FIG. 1, but showing alternate embodiments of the diesel exhaust gas treatment system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A diesel exhaust gas aftertreatment system 10 is provided to treat the exhaust 12 from a diesel combustion process 14, such as a diesel compression engine 16. The exhaust 12 will typically contain oxides of nitrogen (NO_(x)) such as nitric oxide (NO) and nitrogen dioxide (NO₂) among others, particular matter (PM), hydrocarbons, carbon monoxide (CO), and other combustion byproducts.

The system 10 includes a burner 18 that selectively supplies the exhaust 12 at an elevated temperature to the rest of the system 10, a diesel particulate filter (DPF) 20 connected downstream from the burner 18 to receive the exhaust 12 therefrom, and a NO_(x) reducing device 22, such as a selective catalytic reduction catalyst (SCR) 24, as shown in FIG. 1, or a lean NO_(x) trap 26, as shown in FIG. 2, connected downstream from the DPF 20 to receive the exhaust 12 therefrom. To overcome the lower operating temperatures in the exhaust 12 of lean-burn engines, such as the diesel compression engine 16, an active regeneration process for the DPF 20 is employed wherein fuel is ignited in the burner 18 to create a flame 28 that heats the exhaust 12 to an elevated temperature that will allow for oxidation of the PM in the DPF 20. Additionally, in connection with such active regeneration, or independent thereof, the burner 18 can be used in a similar manner to heat the exhaust 12 to an elevated temperature that will enhance the conversion efficiency of the SCR 24. Advantageously, the burner 18 can provide such elevated temperatures, either selectively or continuously, independent of any particular engine operating condition, including operating conditions that produce a low temperature (<300 C) in the exhaust 12 as it exits the engine 16. Thus, the system 10 can be operated without requiring adjustments to the engine controls.

The burner 18 preferably will include one or more injectors 30 for injecting suitable fuel, a couple examples of which are hydrogen and hydrocarbons, and an oxygenator, such as air, to be ignited together with unburned fuel already carried in the exhaust by one or more igniters, such as spark plugs 32. In this regard, each injector 30 can either be a combined injector that injects both the fuel and oxygenator, or a specific injector for one of the fuel or the oxygenator. Preferably, a control system, shown schematically at 34, is provided to monitor and control the flows through the injectors 30 and the ignition by the igniters 32 using any suitable processor(s), sensors, flow control valves, electric coils, etc.

The DPF 20 can be of any suitable construction or type, many of which are known.

Any suitable catalyst can be utilized for the SCR 24, examples of which include Cu based, Iron based and Vandia based catalysts of any suitable construction or type. Preferably, the system 10 also includes an reductant injector 36, again of any suitable construction and type, that can introduce a nitrogenous reductant, such as ammonia, urea, hydrocarbons, hydrogen, or syngas into the exhaust 12 to reduce the NO_(x) content in the exhaust 12 by preferably at least 25% and by as much as 99% under the right conditions. In this regard, the temperature in the SCR will be highly dependent upon the type of reductant used. The injector 36 can be supplied by a pressurized reductant source (not shown) and controlled by the controller 34 or an independent controller (not shown).

With reference to FIG. 2, any suitable construction and type of lean NO_(x) trap 26 can be utilized and preferably will store NO₂ during operating conditions that utilize a lean fuel-air mixture, and reduce the stored NO₂ to N₂ and O₂ under operating conditions that utilize a rich fuel-air mixture. In this regard, while not preferred, if required supplemental hydrocarbon fuel can be injected upstream of the trap 26 to produce a rich fuel-air condition in the trap 26 to assist in forming N₂, H₂O and CO₂. from the stored NO₂.

In operation, the need for active regeneration of the DPF 20 by the system 10 can be determined by based on a number of parameters or combination of parameters, such as the DPF pressure drop, DPF soot mass, a predetermined operating time set point, and fuel consumption rate. Similarly, the active regeneration of the DPF 20 can be terminated based on a number of parameters or combination of parameters, such as the DPF pressure drop, DPF soot mass, and a predetermined regeneration time set point. During active regeneration, the injection of fuel and air via the injector(s) 30 can be based on a number of parameters or combination of parameters, including the flow rate of the exhaust 12, the oxygen concentration in the exhaust 12 and, the inlet and outlet temperatures of the exhaust 12 to and from the DPF 20, with flame stability being monitored by igniter ionization detection or by comparing the inlet and outlet temperatures of the exhaust 12 to and from the burner 18.

Similar control schemes utilizing the corresponding and suitable parameters for the SCR 24 and/or lean NO_(x) trap 26 can be utilized to provide active use of the burner 18 to improve performance and/or provide regeneration.

It should be appreciated that the system 10 can provide enhanced fuel efficiency in comparison to known aftertreatment systems that require excess fuel injection into the engine or system in order to obtain suitable regeneration of a DPF. It should also be appreciated that the burner 18 can be designed for a relatively low pressure drop in the exhaust 12 through the burner 18, particularly in comparison to systems that rely on passive or active regeneration by passing the exhaust through a DOC upstream of a DPF to provide sufficient NO₂ for passive regeneration of the DPF. It should further be appreciated that the passage of the exhaust 12 through the DPF 20 upstream of the SCR 24 tends to dampen the thermal fluctuations in the SCR 24 which can simplify the control of the reductant injection.

With reference to FIG. 3, an alternate embodiment of the system 10 is shown wherein a DOC 40 is connected between the burner 18 and the DPF 20 to provide NO₂ in the exhaust 12 for passive regeneration of the DPF 20 at some level during operating conditions that are favorable to passive regeneration. This can reduce the demand for active regeneration by the burner 18 and thereby increase the overall fuel efficiency of the system 10.

FIG. 4 shows yet another embodiment of the system 10 similar to FIG. 3, but having yet another DOC 42 added between DPF 20 and the SCR 24 to provide additional NO₂ to optimize the reactions in the SCR 24. As further alternative for the system 10, a fuel injector 44 can be added between the burner 18 and the DOC 40 to selectively provide additional fuel, two examples of which are hydrocarbon fuel and hydrogen, to enhance the reactions in the DOC 40 and produce additional quantities of NO₂ in the exhaust under certain operating conditions. 

1. A diesel exhaust gas treatment system to treat the exhaust from a diesel combustion process, the system comprising: a burner to receive the exhaust and selectively heat the exhaust with a flame to supply the exhaust at an elevated temperature to the rest of the system; a diesel particulate filter connected downstream from the burner to receive the exhaust therefrom; and at least one of a selective catalytic reduction catalyst and a NO_(x) trap connected downstream from the diesel particulate filters to receive the exhaust therefrom
 2. The system of claim 1 further comprising a diesel oxidation catalyst connected downstream from the burner to receive the exhaust therefrom and upstream from the diesel particulate filters to deliver the exhaust thereto.
 3. The system of claim 2 further comprising a diesel oxidation catalyst connected downstream from the diesel particulate filters to receive the exhaust therefrom and upstream from the selective catalytic reduction catalyst to deliver the exhaust thereto.
 4. The system of claim 2 further comprising a fuel injector located downstream from the burner and upstream of the DOC.
 5. The system of claim 1 wherein the burner comprises at least one fuel injector and at least one igniter.
 6. The system of claim 1 wherein the at least one of a selective catalytic reduction catalyst and a NO_(x) trap is a selective catalytic reduction catalyst and further comprising a reductant injector connected upstream from the catalyst.
 7. A method of treating a diesel exhaust from a diesel combustion process, the method comprising the steps of: (a) selectively increasing the temperature of the exhaust by burning a fuel in the exhaust flow downstream from the diesel combustion process; (b) removing soot from a filter by oxidizing carbon into the increased temperature exhaust provided from step (a); and (c) removing NO_(x) carried in the exhaust provided from step (b).
 8. The method of claim 7 further comprising the step of producing NO₂ by passing the exhaust from step (a) through an oxidation catalyst prior to step (b).
 9. The method of claim 8 further comprising the step of producing NO₂ by passing the exhaust from step (b) through an oxidation catalyst prior to step (c).
 10. The method of claim 8 further comprising the step of injecting fuel into the exhaust after step (a) and prior to step (b).
 11. The method of claim 7 wherein step (a) comprises the steps of injecting a fuel into the exhaust and igniting the fuel.
 12. The method of claim 7 wherein step (c) comprises converting NO_(x) to N₂ by passing the exhaust over a selective catalytic reduction catalyst.
 13. The method of claim 7 wherein step (c) comprises trapping NO_(x). 